It's Kuba or Kubuś.Gocha is an augmentative, really. It might be used in a friendly manner (assuming the lady doesn't mind) but you wouldn't call a child like that.

As for most common names, it pretty much depends on the generation. For example, Antoni, Jan, Jakub are extremely popular nowadays but it's their comeback after decades - pretty rare among people in their 20s -50s (especially the first two).